To carry out geometric inspection of vehicles, there is usually secured to each wheel an individual measurement housing of known type.
French publication 2.304.057 discloses an apparatus for the inspection of parallelism of the front wheels of a vehicle, which comprises two casings of known type interconnected by a resilient connection member.
EP 015.826 discloses a geometric inspection apparatus for vehicles comprising four casings permitting obtaining six emissions and six receptions of laser beams.
The unpublished U.S. patent application Ser. No. 08/320242 filed Oct. 11,1994, by the applicant also discloses an example of casing permitting obtaining the angles characteristic of the geometry of the vehicle.
At the outset, the apparatus for geometric measurement of the angles of wheels, of parallelism, of camber, of steering axis inclination and of pivoting were obtained by means of two measurement casings which were moved from one axle to the other to effect geometric measurement. This arrangement of the prior art had the drawback that it was necessary to dismount, then to remount the measurement casings from one axle to the other, in which case there were errors due to mounting and required frequent calibration operations.
The art has thus evolved toward the use of individual casings, each measurement casing being secured to a corresponding wheel of the vehicle, the casings being interconnected by resilient members connecting for example the electrical, electro-magnetic or the like angular receivers of known type.
Thanks to this arrangement, it was possible to note the following geometric characteristics of the vehicle: front and rear angles of parallelism, angles of orientation of each individual wheel relative to selected reference axes (the reference axes chosen are generally the axis of symmetry of the vehicle which corresponds to the mid line of the front and rear axles and the axis of geometry of the vehicle which corresponds to the bisector of the angles of orientation of the rear wheels).
To obtain complete measurements, so as to determine the pivot angles and angles of steering axis inclination, it proved to be necessary to provide in the measurement casings of the front steered wheels inclinometers thanks to which there would be carried out the following measurement method: the front steered wheels are turned to the left and then to the right while measuring the turning angles corresponding to each direction of turning, there are measured simultaneously the variations of the angles by means of inclinometers and the angles of inclination of the steering axes and of pivoting are determined by computation.
In the known systems, pivoting plates permitting turning the front wheels of the vehicle do not ordinarily comprise receivers of angles supplying directly the turning angle of each steering wheel. This drawback is overcome in known systems by using measurements supplied by six angular receivers, in which four are disposed in the two casings of the front steerable wheels and of which two are disposed in the two casings of the unsteered wheels, and by computing from the measurements of angles obtained the turning angles of the steered wheels by means of a computation system with a microprocessor of known type.
Because of this, in this arrangement, the two casings of the front wheels comprise two arms which extend longitudinally toward the front of the vehicle. When the front steerable wheels are turned, the ends of these arms have a slight vertical clearance movement from the geometric arrangement of the front wheel, which results in a measurement error or a substantial deviation, in particular because of the fact that the angles of steering axis inclination and of pivoting of the wheel are not zero. In an extreme case, a substantial vertical clearance gives rise to contact at the end of the angular receiver located at the end of a casing arm of a front wheel, this contact completely nullifying the measurements. In the other case, when the entire geometric inspection apparatus is disposed on a raised bridge, the elements of the raised bridge which are located in the space comprised between the two arms mentioned above are also susceptible to interfere with the operation of the angular arm receivers and to nullify these measurements: this is the case in particular for the safety bars and contours located in front of the path of the vehicle and adapted to avoid any untimely movement of the vehicle.
Similarly, in the case of vehicles having an aerodynamic geometry and a very low shock absorber or a spoiler, it is not possible to provide a transverse resilient connection between the receivers of the front arms or to provide for the passage of an optical measuring beam.